This invention relates to an optoelectronic device (semiconductor optical module) and a manufacturing method thereof and, more in particular, it relates to a technique effective for applying a technique of mounting a semiconductor laser chip, a photodetector and an optical fiber on a main surface of a platform referred to as a silicon platform and covering such optical parts with a guard layer r such as ma de of transparent silicone gel.
An optoelectronic device incorporated with a semiconductor laser (semiconductor laser chip) has been used as a light emitting source for an information processing device or as a light emitting source e for optical communication.
xe2x80x9cPaper s C-3-33 of the IEICE collection for 1999 Electronic Society Conference of the Institute of Electronics Information and Communication Engineers of Japanxe2x80x9d published from the Institute of Electronics Information and Communication Engineers of Japan discloses a surface mount type LD module according to Bellcore standards adopting a passive alignment system. In the surface mount type LD module, optical fibers are fixed with UV epoxy adhesives, and an optical device and an optical coupling portion are potted with an Si series resins. Also, the literature describes data for change of optical power in a temperature cycle test for xe2x88x9240 to 80xc2x0 C.
Further, a passive alignment mount using the silicon platform is described in xe2x80x9cPapers SC-2-77 of the IEICE collection for 1996 General Conference of the Institute of Electronics Information and Communication Engineers of Japanxe2x80x9d published from the Institute of Electronics Information and Communication Engineers of Japan. The latter literature describes fixing of the LD module.
In the manufacture of conventional optoelectronic devices (semiconductor optical module) using silicon platforms (platforms), the top end of an optical fiber fitted along a groove of a silicon platform is adjusted for optical coupling with a semiconductor laser chip fixed to the surface of the silicon platform and then an optical fiber fitted to the silicon platform is fixed with a thermosetting resin (thermosetting epoxy resin) or an adhesive such as UV-ray setting adhesive.
The present applicant has made a study on the technique of fixing the optical fiber to a silicon platform in a short period of time and a technique of suppressing the change of optical power.
The present applicant has already proposed a technique of fixing the optical fiber to a silicon platform in a short period of time (Japanese patent application No. Hei 9-78602 filed on May 28, 1997) (Japanese Patent Laid-Open (Kokai) No. Hei 10-268168). In this technique, the existent method of preparing a silicon platform (platform) having a groove on the surface, then fixing an optoelectronic transducer (semiconductor laser chip) on the surface of the silicon platform at one end of the groove and fitting an optical fiber along the groove and, subsequently, controlling the light transmitting/receiving state between the optoelectronic transducer and optical fiber and fixing the optical fiber to the platform by means of a thermosetting resin is modified such that the optical fiber is provisionally fixed in a state being urged to the platform by a fixing means having shorter fixing time than the curing time of the thermosetting resin and then pressure is removed and the optical fiber is finally fixed by the thermosetting resin.
As an example, after coating a UV-ray setting adhesive to the optical fiber and the platform portion, the UV-ray setting adhesive is set by the irradiation of UV-rays thereby provisionally fixing the optical fiber to the platform and, subsequently, a portion of the optical fiber with the distance from the semiconductor laser chip remote from the provisional fixing position is covered with a thermosetting resin.
In this technique, since the provisional fixing has been completed by the UV-ray setting adhesive, after thermosetting resin is applied, the platform can be moved even before curing of the thermosetting resin, so that the platform can be removed from the fiber fixing machine in a short period of time and curing treatment (main fixing) of the thermosetting resin can be conducted by a batch processing. The batch processing can shorten the fixing time of the optical fiber per platform. Further, the optical coupling has high reliability.
On the other hand, the present applicant has adopted the technique and also made a study on the technique of reducing the package cost. Then, to reduce the package cost, the package main body (casing) and a lid (cap) constituting the package are made of plastic material and a structure of bonding the casing and the cap with a resin was adopted. Further, since the plastic material has less moisture proofness compared with ceramic material, it is intended to improve the moisture proofness by sealing a transparent silicone gel in the casing thereby covering the surface for each of parts including a semiconductor laser chip on a platform with the silicone gel.
However, it has been found by the present applicant that the fixing strength of the optical fiber is lowered and the reliability for fixing is lowered and the moisture proofness is lowered in the silicone gel sealing structure. It has been found that this is attributable to voids formed in the silicone gel.
Further, as a result of experiment and study on the mechanism of forming voids, it has been found that initially formed voids are increased in the subsequent temperature cycle, that is, depending on the temperature of the working circumstances.
Then, the present applicant has already proposed a technique of manufacturing an optoelectronic device having high fixing strength and high fixing reliability of optical fibers (Japanese Patent Application No. Hei 10-270339 filed on Sep. 24, 1998) (Japanese Patent Laidopen (Kokai) No. 2000-137147).
The followings are disclosed in the proposal described above (Japanese Patent Application No. Hei 10-270339) which concerns the problem of voids formed in the silicone gel.
FIG. 8 is a view schematically illustrating a portion in which an optical fiber 3 is fixed in a groove 2 of a silicon platform (platform) 1 by primary fixing (provisional fixing) with a UV-ray setting adhesive 4 and by secondary fixing (main fixing) with a thermosetting resin 5 and the upper surface of the silicon platform 1 is covered with a silicone gel 6. The optical fiber 3 comprises a clad 3b and a core 3a situating at the center thereof. A semiconductor laser chip 7 is shown by a dotted chain. Voids 10 tend to be formed, as shown in FIG. 8, in a silicone gel in a surrounded region 9 formed with a groove surface forming the groove 2 of the silicone platform 1 and the optical fiber.
Presence of the voids lowers the fixing strength of the optical fiber 3 to the silicon platform 1 and lowers the reliability of the fixing.
Further, the presence of the voids 10 per se lowers the moisture proofness and, when water intrudes, it is trapped around the void as the seed, so that water is less released to the outside to deteriorate the moisture proofness. Since the semiconductor laser chip 7 and the photodetector are disposed on the extended top of the optical fiber, and a wiring layer or wire is present at the periphery, trapping of water to the voids 10 may possibly cause oxidation or corrosion in each of the portions and lowers the moisture proofness of the optical module.
Further, in a state where water is trapped to the voids, water is frozen in a case where the optical module is exposed to the temperature lower than the freezing point and there may be also a worry of trouble caused by the volumic change.
In the experiment, as shown in FIGS. 9(a),9(b) and FIGS. 10(a),10(b), a metal frame 16 is placed at the bottom of a vessel 15, on which two glass capillaries 17 (0.13 mm inner diameter) are arranged in parallel and in contact with each other, and silicone gel 6 is filled in the vessel 15 such that the silicone gel 6 covers throughout the surface and the inside of both of the capillaries 17 so as not to involve voids. Subsequently, the vessel 15 was placed under the cure bake condition (baking temperature at 120xc2x0 C., baking time for 60 min).
FIGS. 9(a),9(b) is a schematic view illustrating the state of distribution of voids 10 in a state where the silicone gel is cured under the cure bake condition in which FIG. 9(a) is a plan view and FIG. 9(b) is a cross sectional view.
Further, after curing of the silicone gel, an environmental test such as for temperature cycle was conducted. The environmental test such as for temperature cycle conducts (1) 40 cycles of temperature change for xe2x88x9240 to +85xc2x0 C. as one cycle in about 35 min, (2) 136 hours of exposure to high temperature high humidity (85xc2x0 C., relative humidity 85%), (3) 30 min of high temperature baking (120xc2x0 C.) and (4) 1.5 hours of leaving at low temperature (55xc2x0 C.), successively in this order.
FIGS. 10(a),10(b) is a schematic view illustrating the distribution of voids 10 formed in the silicone gel by the environmental test such as for temperature cycle in which FIG. 10(a) is a plan view and FIG. 10(b) is a cross sectional view.
Voids 10 in FIGS. 9(a),9(b) and FIG. 10(a),10(b) are depicted based on photograph and their positions are exact even when the shape is somewhat different from the actual state.
As shown in FIGS. 9(a),9(b), the voids 10 are scattered in the inner diametrical portion of the capillaries 17 but are not present on both ends thereof. It is considered that the silicone gel can be freely moved in the inner diametrical for the inside and the outside of the capillaries 17 in the inner diametrical portion on both ends of the capillaries 17 (open space), while movement of the silicone gel for compensating the shrinking decrease of the volume is insufficient at the inner diametrical portion inside the capillary 17, so that gaps, namely voids 10 are formed.
Further, as shown in FIGS. 10(a),10(b), since the they are exposed repeatedly to varying temperature and humidity in the environmental test, spaces are newly formed along with movement of the silicone gel to increase the voids 10 and, in addition, the shape of the voids 10 deforms by joining or separation of the neighboring gaps to each other. The voids increase in the size and are greatly displaced positionally at a high temperature of 120xc2x0 C., whereas a number of small voids are formed at a low temperature of xe2x88x9255xc2x0 C.
As shown in FIGS. 10(a),10(b), it can be seen that voids are newly formed also in a region where the voids 10 were not present in FIGS. 9(a),9(b), that-is, in a region surrounded with the metal frame 16 and the two capillaries 17 (surrounded region 9).
On the other hand, when a silicone gel is sealed in the plastic casing to cover the surface of each of parts including a semiconductor laser chip 7 on a silicon platform 1, as shown in FIG. 11, it has been found that voids 10 are formed not only in a silicone gel 6 filled in a groove 2 below an optical fiber 3 but also, as shown in FIG. 12 and FIG. 13, voids 10 are formed between the top end of the optical fiber 3 and the semiconductor laser chip 7.
It is considered that since the gap between the top end of the optical fiber 3 and the front facet of the semiconductor laser chip 7 is as narrow as 40 to 50 xcexcm or less, the portion does not act as an open space but the voids are liable to be formed, particularly, in a case of undergoing heating repetitively. That is, in an initial state where the silicone gel is filled after fixing and curing processing has been applied for the silicone gel 6, formation of the void 10 is not observed between the top end of the optical fiber 3 and the front facet of the semiconductor laser chip 7. However, after the heat cycle test, it sometimes occurs that voids 10 are formed between the top end of the optical fiber 3 and the front facet of the semiconductor laser chip 7.
In a state where the void 10 is formed between the top end of the optical fiber 3 and the semiconductor laser chip 7 and the void 10 interferes an optical channel of a laser beam 11 emitted from the semiconductor laser chip 7 (refer to FIG. 12, FIG. 13), since the void 10 acts as a lens, the direction of the laser beam 11 emitted from the semiconductor laser chip 7 is changed (deviated to cause shading) to sometimes inhibit optical coupling with the optical fiber 3 or lower the optical coupling efficiency. The optical coupling is often inhibited when the optical fiber 3 is a fine single mode fiber with the core diameter 3a of about 10 xcexcm diameter. In FIG. 11 through FIG. 13, a photodetector 19 receives a laser beam 11 emitted from the rear facet of the semiconductor laser chip 7. Further, in FIG. 13, the silicone gel 6 is sometimes present over the entire upper surface of the platform 1.
By the way, as a result of analyzing and studying a monitor current Is of the photodetector 19 for receiving (monitoring) a laser beam 11 emitted from the rear facet of the semiconductor laser chip 7, it has been found that Is tracking characteristic shows characteristic with hysteresis in some products as shown in FIG. 14. In the graph, the temperature is expressed on the horizontal axis and the relative value (xcex94Pf) for the output is taken on the vertical axis. xcex94Pf is a value obtained by subtracting a Pf value at 25xc2x0 C. from a Pf value at txc2x0 C. and dividing the subtracted value by the Pf value at 25xc2x0 C.
As shown in the graph, xcex94Pf is determined by (1) lowering the temperature from 25xc2x0 C. to xe2x88x9240xc2x0 C. successively, (2) elevating the temperature from xe2x88x9240xc2x0 C. to 85xc2x0 C. successively and then (3) lowering the temperature from 85xc2x0 C. to xe2x88x9240xc2x0 C. successively. While the xcex94Pf value is to be substantially identical both in the cases of (1), (2) and (3), it has been found that xcex94Pf draws a hysteresis loop of abruptly increasing in (2) after (1) and then returning to a lower value at about 50xc2x0 C. in some products.
As a result of analysis and study in this regard, the present inventors have found that voids 10 are present between the rear facet of the semiconductor laser chip 7 and the light receiving facet of the photodetector 19 and xcex94Pf fluctuates by the presence of the void 10 and, depending on the case, Is tracking characteristic forms hysteresis.
FIG. 15 is a schematic view showing a semiconductor laser chip (LD) 7, a photodetector (PD) 19 and an optical fiber 3 fixed on the main surface of the platform 1, and a silicone gel 6 covering the main surface of the platform 1. As shown in the figure, a void 10 is formed on the rear facet of the semiconductor laser chip 7. In a state where the void 10 interferes the optical channel of a laser abeam 11 emitted from the rear facet of the semiconductor laser chip 7, since the void 10 acts as a lens, the direction of the laser beam 11 emitted from the semiconductor laser chip 7 is changed (deviated to cause shading), to fluctuate the monitor current Is.
As shown in FIG. 15, the present applicant adopts a structure for the semiconductor laser chip 7 that the end of the bonding layer 41 for fixing the semiconductor laser chip 7 on the platform 1 recedes inward of the facet 43 for emitting the laser beam. This structure is adapted to prevent the end of the bonding layer 41 from protruding into the optical channel to lower the amount of light to be transmitted or inhibit the transmission at all by shielding when the end of the bonding layer 41 is raised exceeding the facet 43.
The bonding layers 41, 42 are formed by metallizing a solder on the main surface of the platform 1. Then, after stacking the semiconductor laser chip 7 and the photodetector 19 such that their respective lower electrodes 45, 46 are overlapped on the bonding layers 41, 42, the bonding layers 41, 42 are melted by heating to fix the semiconductor laser chip 7 and the photodetector 19. The length of the lower electrodes 45, 46 are slightly longer than the length for the semiconductor laser chip 7 or the photodetector 19, so as to reliably fix the lower electrodes 45, 46 to the platform 1. Accordingly, the ends of the lower electrodes 45, 46 may further recede sometimes further inward of the ends of the bonding layers 41 and 42.
The receding length of the lower electrodes 45, 46 from the emitting facet 43 or receiving facet 44 is not limited particularly but it is defined as, for example, from 10 to 40 xcexcm in view of variation in the length of the semiconductor laser chip 7 due to the scattering for the cleaving position upon forming the semiconductor laser chip 7.
Further, the distance from the main surface of the platform 1 to the surface of the semiconductor laser chip 7 displaced from the lower electrode 45 is not particularly limited and it is determined, for example, as 4 to 7 xcexcm in view of the structure that the stripe portion (light emission portion) slightly protrudes.
Then, in the surrounded region 9 formed by the receding of the lower electrode 45 and the bonding layer 41, the void 10 may occasionally be formed in that the silicone gel can not move freely upon contracting in curing of the silicone gel 6.
In FIG. 15, the state of forming the void 10 is illustrated on the rear facet of the semiconductor laser chip 7 but the void 10 may possibly be formed also on the light receiving facet of the photodetector 19.
On the emitting facet of the semiconductor laser chip 7 opposed to the end face of the optical fiber 3, since the main surface of the platform 1 is deeply recessed, formation of the void 10 can be suppressed.
The void 10 may be possibly moved in the temperature cycle test to protrude into an optical channel between the semiconductor laser chip 7 and the photodetector 19 to fluctuate the monitor current Is of the photodetector 19.
In view of the above, the present inventor has studied a structure of a platform causing less voids between the semiconductor laser chip 7 and the photodetector 19 and, as a result, has accomplished this invention.
This invention intends to provide an optoelectronic device capable of stably monitoring the. optical power not hindering transmission (passage) of a laser beam from the semiconductor laser chip to the photodetector, as well as a manufacturing method thereof.
Further, this invention intends to provide an optoelectronic device capable of stably outputting optical power and stably monitoring the optical power, as well as a manufacturing method thereof.
The outline for typical aspects of the invention disclosed in this application is to be explained simply as below.
(1) The invention provides an optoelectronic device in which a semiconductor laser chip (light emitting part), an optical fiber (light receiving part) for taking at the front end a front laser beam emitted from the front facet of the semiconductor laser chip, and a photodetector (light receiving part) for receiving a rear laser beam emitted from the rear facet of the semiconductor laser chip are fixed on a main surface of a platform (silicon platform), and each of the optical parts and portions including the optical channels between each of the optical parts are covered with silicone gel, wherein the main surface portion of the platform between the top end of the optical fiber and the semiconductor laser chip and between the photodetector and the semiconductor laser chip has concaves so that voids are not formed upon curing of the silicone gel. The edge of the concave between the semiconductor laser chip and the optical fiber, on the side of the semiconductor laser chip (light emitting part), is situated closer to the semiconductor laser chip than to the end face (front facet) of the semiconductor laser chip, and the edge of the concave between the semiconductor laser chip (light emitting part) and the photodetector element (light receiving part), on the side of the semiconductor laser chip, is situated closer to the semiconductor laser chip than to the end face (rear facet) of the semiconductor laser chip.
Further, the ends of the bonding portion for fixing the semiconductor laser chip and the photodetector to the platform recede inward of the emitting facet and the light receiving facet respectively so as not to cause disadvantage in the transmission of light due to the protrusion of the end of the bonding layer. Further, the length of the edge for the concave extending in the direction perpendicular to the optical channel in-the concave between the light emitting part and the photodetector is made smaller than the width of the light emitting part opposed thereto.
The optoelectronic device as described above is manufactured by the following method.
A method of manufacturing an optoelectronic device having a casing, a cap covering the casing, a platform attached in the casing, a light emitting part fixed to the main surface of the platform and for emitting an optical beam from the end face, a light receiving parts fixed on the main surface of the platform and for receiving the optical beam emitted from the light emitting part, and a guard layer made of a transparent resin, filled within the casing and for covering the light emitting part, the light receiving parts and optical channels between the light emitting part and the light receiving parts, in which
concaves are present in the main surface of the platform between the light emitting part and the light receiving part, and an edge of each of the concaves on the side of the. light emitting part is present closer to the light emitting part than to the end face of the light emitting part, wherein the method includes:
a step of mounting the light emitting part and the light receiving part on the platform;
a step of attaching the platform to the casing;
a step of connecting between the light emitting part and the light receiving parts, and between predetermined portions of the platform with conductive wires;
a step of filling the transparent resin in the casing;
a step of leaving the entire case in an atmosphere at a predetermined vacuum degree for a predetermined period of time to defoam voids in the transparent resin;
a step of leaving the case entirely in an atmosphere at a predetermined heating temperature for a predetermined period of time thereby curing the transparent resin to form a guard layer made of the transparent resin; and
a step of attaching the cap to the casing.
The end of the bonding layer for fixing the light emitting part to the platform is fixed so as to recede inward of a light emitting facet of the light emitting part, and the end of the bonding layer for fixing the light receiving part to the platform is fixed so as to recede inward of a light receiving facet of the light receiving part.
The semiconductor laser chip as the light emitting part is fixed on the platform, a photodetector as the light receiving part is fixed on the platform so as to receive an optical beam emitted from the rear facet of the semiconductor laser chip, an optical fiber as the light receiving part is fixed on the platform for taking a optical beam emitted from the front facet of the semiconductor laser chip, the case as the casing is made of a plastic material with a guide for guiding the optical fiber, the transparent resin is filled in the casing so as to cover the platform, the semiconductor laser chip, the photodetector and the top end of the optical fiber, the vacuum defoaming treatment is conducted, then the thermosetting treatment to the transparent resin is conducted to form a transparent guard layer and the cap made of the plastic material is attached to the casing so as to cover the opening of the casing.
Any one of resin selected from silicone gel, silicone rubber, low stress epoxy resin, acrylic resin and urethane resin is filled in the casing as the transparent resin for forming the guard layer.
According to the means (1) above, (a) a concave is formed on the main surface portion of the platform between the semiconductor laser chip and the photodetector to form an open space, the edge of the concave on the side of the semiconductor laser chip is situated closer to the semiconductor laser chip than to the end face (rear facet) of the semiconductor laser chip, so that voids in the silicone gel formed in the fixed portion of the semiconductor laser chip in-adjacent with the open space are more effectively defoamed and no more result in troubles in transmitting and receiving of the laser beam between the semiconductor laser chip and the photodetector. Accordingly, the monitor current Is of the photodetector less fluctuates and stable laser beam intensity monitoring can be attained.
(b) A concave is formed also in the portion of the main surface of the platform between the semiconductor laser chip and the top end of the optical fiber to form an open space, the edge of the concave on the side of the semiconductor laser chip is situated closer to the semiconductor laser chip than to the end face (front facet) of the semiconductor laser chip, so that voids in the silicone gel formed in the fixed portion of the semiconductor laser chip near the open space are more effectively defoamed and cause no trouble in the transmitting and receiving of the laser beam between the optical fiber and the semiconductor laser chip. Further, since also the top end of the optical fiber forms an open space by the concave, voids are not situated in the optical channel at the top end of the optical fiber. Accordingly, the laser beam emitted from the emitting facet of the semiconductor laser chip can be received efficiently to the optical fiber and stable optical communication is possible.